Various kinds of force may be measured by subjecting certain kinds of resistive elements to that force. Those resistive elements have an electrical resistance which varies as a function of the applied force. Examples of such resistive elements are strain gauges and piezoresistive elements. The resistance of the element may be determined, and thus the force may be measured, by applying electrical power to the resistive element and monitoring an electrical output signal related to the power dissipated in the resistive element, for example, a signal related to the voltage across the resistive element or the current flowing in the resistive element. The magnitude of the electrical output signal is related to the amount of power supplied to the resistive element. In the past, the magnitude of the electrical output signal has been inadequate for direct connection of that signal to processing circuitry usually associated with force transducers, for example, a microcomputer or other computer circuitry.
Accordingly, it has been proposed that a signal related to the voltage across the resistive element or the current flowing in the element be amplified before it is applied to the processing circuitry. This is unsatisfactory for several reasons. Amplifiers add to circuit complexity making the transducers prone to failure. Amplifiers take up needed space, which make designing a compact transducer difficult. Amplifiers have a tendency to drift which would render the force measurement inaccurate over time if the drift were not taken into account. Taking drift into account adds more unneeded complexity to the circuit design. Amplifiers also may add various forms of undesirable instability to the circuitry making force measurement difficult or impossible. Amplifiers are prone to electrical noise and are influenced by electromagnetic radiation, particularly if the amplifiers are situated on a circuit board outside the transducer. Amplifiers are thus not an attractive solution to the problems associated with low output signal levels from resistive force transducers.
A need, therefore, exists to provide adequate output signal levels from resistive force transducers without introducing the problems associated with signal amplification. It thus is one object of the invention to provide a transducer apparatus for measuring force applied to the transducer apparatus without the need for amplifying an electrical output signal produced by the transducer, such electrical output signal having a sufficient magnitude so that it can be directly applied to processing circuitry associated with the transducer apparatus. Additional objects of the invention are apparent from the advantages of the invention described in the detailed description below.